In recent years, to reduce CO2 and emissions, there has been an increase in research relating to internal-combustion engines into supercharged lean, a large amount EGR, and premixed self-ignition combustion. According to the research, a stable combustion state near the combustion limit is required in order to reduce CO2 and emissions most effectively. In addition, while petroleum-based fuel dwindles, the robustness that allows stable combustion even with various fuel such as biofuel is required. The most important point to achieve such stable combustion is to reduce variations in ignition timing of an air-fuel mixture and smooth combustion that burns out the fuel during an expansion stroke.
In addition, an in-cylinder injection system that directly injects fuel into a combustion chamber is employed for a fuel supply in internal-combustion engines to improve transient responsiveness, improve volumetric efficiency by a latent heat of vaporization, and achieve significantly-retarded combustion for catalyst activation at low temperature. However, adoption of the in-cylinder injection system promotes combustion fluctuation due to oil dilution caused by crash of sprayed fuel against a combustion chamber wall with remaining droplet and degradation in fuel atomization due to deposits produced around an injection aperture of an injection valve by liquid fuel.
To prevent such oil dilution and degradation in fuel atomization caused by adoption of the in-cylinder injection system and reduce a variation in ignition timing to achieve stable combustion, it is important to atomize fuel spray so that the fuel in the combustion chamber smoothly vaporizes.
As a method of atomizing the fuel spray injected from a fuel injection valve, there has been known a method using a shear force of a thinned liquid film or cavitation occurring by separation of a flow, or atomizing fuel adhering to a surface by mechanical vibration of ultrasonic waves.
Patent Document 1 discloses a fuel injection nozzle that causes the fuel passing through a spiral passage formed between a wall surface of a hollow hole in a nozzle body and a sliding surface of a needle valve to be a rotating flow in a fuel basin that is a circular chamber. This fuel injection nozzle injects the fuel rotating in the fuel basin from a single injection aperture that is located downstream of the fuel basin and has a divergent tapered surface. The injected fuel is dispersed, and mixing with air is promoted.
Patent Document 2 discloses a fuel injection valve that injects fuel mixed with air bubbles generated by a difference between pressures in an air bubble generating passage and an air bubble retaining passage, and atomizes the fuel by collapse energy of air bubbles in the fuel after the injection.
As described above, various approaches have been suggested for fuel injection nozzles and fuel injection valves.